Restart the device. It'll ask you to put in your password the first time. After that see if it works. If it still doesn't work. You may try backing up your device first and using iTunes to reset it. Test the finger print scanner before you restore your backup. Then test it again after you restore it.

Does it give you an error when placing your thumb on it? Try deleting current saved finger prints and re adding them. And see if t works then.